Air-operated vacuum pumps of the venturi type having unitary bored housings are well known in the art, and reference is made to U.S. Pat. No. 4,158,528 for its disclosure of a preferred embodiment of such a pump which provides an ultra-high vacuum force, equal to about 29.7" mercury, and an air consumption to vacuum flow ratio of about 3.2:1. In other words, such pumps provide the highest possible vacuum force, approaching a perfect vacuum force of 30" mercury, but consume 3.2 volumes of compressed air fed thereto to operate the pump for each volume of air withdrawn from a space or container being evacuated, i.e., vacuum flow.
Vacuum pumps which require a large volume of compressed air for the operation thereof necessitate the use of large capacity air compressors which are expensive, heavy and non-portable.
It is known to design air-operated vacuum pumps having unitary bored housings so as to improve the vacuum flow capacity thereof so that it exceeds the air consumption volume, but such known pumps are not capable of developing a vacuum force greater than about 10" mercury. Moreover, the highest possible vacuum flow air consumption ratio is only about 2.2:1 for pumps producing a vacuum force of about 4.1" Hg. Thus, the vacuum flow capacity is only improved at the expense of substantially reducing the vacuum force capability of the pump, and such pumps are unsatisfactory for use in evacuations requiring the development of a high vacuum force, i.e., greater than about 25" Hg.
It is also known to use electric vacuum pumps which have a large vacuum flow capability. However such pumps are large, heavy, expensive and noisy during operation. Moreover they have movable mechanical components which require maintenance and replacement.